1. Field of the Invention
The present invention relates generally to a wireless packet data communication system, and in particular, to a method and apparatus for transmitting/receiving a Channel Quality Indicator (CQI) feedback channel in a wireless packet data communication system.
2. Description of the Related Art
Generally, the mobile communication system is evolving from the early voice communication system for mainly providing voice services into the high-speed, high-quality wireless packet data communication system for providing data services and multimedia services. Recently, various mobile communication standards such as High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) by the 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) by the 3rd Generation Partnership Project 2 (3GPP2), Institute of Electrical and Electronic Engineers (IEEE) 802.16, etc. have been developed to support high-speed, high-quality wireless packet data transmission services.
The current 3G wireless packet data communication system, such as HSDPA, HSUPA, HRPD, etc., uses an Adaptive Modulation and Coding (AMC) technique and a channel-sensitive scheduling technique to improve the transmission efficiency. With use of the AMC technique, a transmitter can adjust the amount of transmission data according to the channel state. That is, the transmitter reduces the amount of transmission data to adjust the reception error probability to a desired level if the channel state is poor, and increases the amount of transmission data to adjust the reception error probability to the desired level if the channel state is good, thereby improving the data transmission efficiency. With use of the above channel-sensitive scheduling resource management method, the transmitter selectively services the user with a good channel state among several users, increasing the system capacity compared to the method of allocating a channel to a single user and servicing the user with the allocated channel. The system capacity increase is referred to as a ‘multi-user diversity gain’. In short, in the AMC technique and the channel-sensitive scheduling technique, the transmitter receives partial channel state information fed back from a receiver, and applies the appropriate modulation and coding technique at the most efficient time determined depending on the received channel state information.
To realize the AMC technique and the channel-sensitive scheduling technique, the receiver should feed back the channel state information to the transmitter. The channel state information the receiver feeds back to the transmitter is referred to as a Channel Quality Indicator (CQI).
Recently, extensive research is being conducted to replace Code Division Multiple Access (CDMA), the multiple access scheme used in the 2nd Generation and 3rd Generation mobile communication systems, with Orthogonal Frequency Division Multiple Access (OFDMA) in the next generation mobile communication system. 3GPP and 3GPP2 have started standardization work on the OFDMA-based Evolved system. It is known that OFDMA, compared to CDMA, can contribute to the greater capacity increase. One of the several causes of the capacity increase in OFDMA is to perform scheduling in the frequency domain (know as ‘frequency-domain scheduling’). As the capacity gain was obtained through the channel-sensitive scheduling technique according to the characteristic that the channel varies with the passage of time, a greater capacity gain can be obtained with the use of the characteristic that the channel varies according to the frequency. However, to support the frequency-domain scheduling, the transmitter should previously have the channel state information for each individual frequency. That is, CQI feedback is needed separately for each frequency, increasing the CQI feedback load.
In the next generation system, research is being conducted to introduce a Multiple Input Multiple Output (MIMO) technology using multiple transmit/receive antennas. The term ‘MIMO’ as used herein refers to a technology for simultaneously transmitting multiple data streams via multiple transmit/receive antennas using the same resources. It is known that transmitting multiple low-modulation order data streams, compared to increasing the modulation order in a good channel state, is a better way to increase the throughput at the same error probability.
In the MIMO technology, the dimension over which an individual data stream is transmitted is called a ‘layer’. A method of separately applying AMC according to the channel state of the layer is efficient in increasing the capacity. For example, Per Antenna Rate Control (PARC) is a technology for transmitting different data streams via every transmit antenna, and here, the layer is the transmit antennas. Multiple transmit antennas undergo different channels, and the PARC technique applies AMC such that a greater amount of data can be transmitted via transmit antennas with a good channel state and a lesser amount of data can be transmitted via transmit antennas with a poor channel state. As another example, there is Per Common Basis Rate Control (PCBRC), and in this technology, the layer is a fixed transmission beam. Therefore, the PCBRC technique transmits a greater amount of data over transmission beams with a good channel state, and transmits a lesser amount of data over transmission beams with a poor channel state.
Space Division Multiple Access (SDMA), a technology for allocating different users to multiple transmission beams, can increase the capacity through space-domain scheduling, as OFDMA can increase the capacity through frequency-domain scheduling.
The MIMO and SDMA technologies are also referred to as Single-User MIMO and Multi-User MIMO, respectively. That is, data streams are transmitted separately over individual layers, and the transmission is classified into Single-User MIMO and Multi-User MIMO depending on whether they head toward a single user or multiple users.
FIG. 1A illustrates the concept of Multi-User MIMO.
Referring to FIG. 1A, an Access Network (AN) 10 transmits a data stream to two Access Terminals (ATs) 11 and 12. Here, because the data stream is transmitted over the same frequency/time resources, it should be transmitted over separated space resources. Therefore, the data stream being transmitted to the access terminal #1 11 is transmitted with one beam 14, and the data stream being transmitted to the access terminal #2 12 is transmitted with another beam 15.
FIG. 1B illustrates the concept of Single-User MIMO.
Referring to FIG. 1B, unlike in Multi-User MIMO, an access network 10 transmits multiple data streams to one access terminal 11. Therefore, the beams 17 and 18 formed by the access network 10 both head for one access terminal 11.
There is a difference between CQI calculation in Single-User MIMO and CQI calculation in Multi-User MIMO. In Single-User MIMO, a receiving access terminal can apply a layer-based interference cancellation technique because it is designed to receive signals of all layers. However, in Multi-User MIMO, the receiving access terminal cannot perform decoding and interference cancellation on the signals of some layers transmitted to another user. Therefore, the access terminal should calculate a CQI with the inter-layer interference cancelled, when the access terminal operates in Single-User MIMO. However, the access terminal should calculate a CQI with the inter-layer interference considered, when the access terminal operates in Multi-User MIMO.
If a scheduler of the access network has a freedom to select operations of Single-User MIMO and Multi-User MIMO, the access terminal should feed back a CQI supporting the both operations. However, the access terminal, when it already applies MIMO, feeds back a CQI of each individual layer (hereinafter referred to as layer-based CQI). Therefore, in the state where the amount of feedback has already increased, if the access terminal feeds back all CQIs supporting the both operations, the feedback overhead may excessively increase.